The use of digital signals for the transmission of various types of information continues to grow in importance. Digital modulation is often an important part of the process of transmitting a digital signal. Digital modulation involves the mapping of bits into symbols, filtering the symbols into desired pulse shapes, and the translation of baseband pulses onto a carrier signal for transmission.
The mapping of bits into symbols involves, e.g., collecting N bits and mapping those bits into 1 of 2.sup.N signal amplitude and phase values. Pulse shaping is used to limit the bandwidth of the signal to be transmitted to the desired channel bandwidth. It can be accomplished by a digital filtering operation and is often implemented as an FIR (finite impulse response filter). A mixing operation is often used to translate an information signal up to the carrier frequency for transmission purposes.
Given the reliability of digital integrated circuits as compared to analog system components there are advantages to moving to modulation systems which use all or mostly digital circuits. U.S. Pat. No. 5,783,974 discusses various digital modulators which use interpolation to increase the number of samples in the signal to be transmitted. While such systems may support a range of interpolation ratios, the supported ranges normally include interpolation ratios which are integer multiples of an input sample rate. For reasons of increased flexibility, it is desirable that at least some modulators be capable of supporting non-integer interpolation rates.
In view of the above discussion, it becomes apparent that there is a need for new and improved methods of implementing modulators and digital modulator circuitry. It is desirable that at least some of the new methods and apparatus support non-integer upsampling rates.